Needle coke is utilized to make graphite electrodes that are heated to high temperatures in steel production. Feedstocks historically utilized for needle coke production include coal-tar pitch, fluidized catalytic cracker decant oil (also known as slurry oil), petroleum vacuum residues, and ethylene tar pitches and solvent-refined coals.
The molecular composition of the coking feedstock utilized has a significant impact on the quality of needle coke subsequently produced. Greater aromatic content and lower sulfur and nitrogen content correspond to higher-quality, needle coke feedstock suitable for production of commercial electrodes.
Coal tar pitches have a high aromatic content exceeding 90 wt. % and a relatively high nitrogen content often exceeding 1 wt. %, but lower sulfur content of approximately 0.5 wt. % or less. As such, Mochida et al. showed that coal tar can be converted to a feedstock for the production of needle coke simply by removing nitrogen compounds by chromatography.
The use of slurry oil as a needle coke feedstock presents a greater challenge, as it typically contains relatively high levels of both sulfur and nitrogen compounds, while also having a lower aromatic content than coal tar pitch (approximately 70 to 80 wt. %). Conventional hydrotreating is typically used for removing both sulfur via hydrodesulfurization (HDS) and nitrogen via hydrodenitrogenation (HDN) from the slurry oil. However, we show herein that eliminating both sulfur and nitrogen compounds to a level suitable to produce a premium needle coke feedstock requires severe hydrotreating conditions that also causes a significant loss of aromatic content that significantly decreases the yield of high-quality needle coke feedstock.
Thus, improved systems and methods are needed for producing needle coke feedstock from slurry oil that can reduce levels of both sulfur and nitrogen compounds in the slurry oil, while largely preserving aromatic content of the slurry oil to increase both the yield and quality of a needle coke precursor that can be converted to premium needle coke product in a delayed coking apparatus.